Semiconductor light-emitting devices, for example, semiconductor light-emitting diodes and semiconductor laser diodes, have been widely used in various applications, such as display, illumination, and communication. An operational process that takes place in a semiconductor light-emitting device is radiative recombination between electrons and holes in an active region. This recombination generates photons, some of which can escape from the body of the device, to constitute output light. A parameter that characterizes a light-emitting device is brightness, which is related to the number of photons emitted within a certain period of time. Different methods, such as increasing the radiative recombination inside the active region, can enhance the brightness of a semiconductor light-emitting device.
Some modern semiconductor light-emitting devices have a quantum-well (QW) structure in the active region, where the thickness of the well is on the order of several nanometers to several tens of nanometers. Control of the thickness is achieved by using state-of-the-art epitaxial technologies such as metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Due to the relatively small thickness of the well, a quantum confinement effect of carriers (both electrons and holes) occurs in the well, which leads to better device performance. However, a single quantum well may not capture all the injected carriers, especially when a large amount of carriers are injected. This limits output power and hence the brightness of a semiconductor light-emitting device. Accordingly, instead of a single-quantum-well active region, a multi-quantum-well (MQW) active region may instead be provided in high-power, high-brightness semiconductor light-emitting devices.
In an MQW semiconductor light-emitting device, the active region consists of a plurality of quantum well layers separated by barrier layers interposed therebetween. An n-type cladding layer is placed at one side of the active region and a p-type cladding layer is placed at the other side of the active region. Electrons and holes are injected into the active region from the n-type cladding layer and the p-type cladding layer, respectively. In the active region, electrons and holes move in opposite directions and may recombine with each other when they reach the same quantum well. Therefore, the electron density is high near the n-type cladding layer and decreases gradually away from the n-type cladding layer. Similarly, the hole density is high near the p-type cladding layer and decreases gradually away from the p-type cladding layer. The decrease of hole density can be more rapid as compared to that of electron density, because the mobility and diffusion coefficient of holes may be smaller than those of electrons. Accordingly, more radiative recombination occurs in quantum wells close to the p-type cladding layer and less in quantum wells close to the n-type cladding layer. When the number of quantum wells is large, e.g., several tens of quantum wells, the situation becomes more severe. The result is, near the n-cladding layer, a lot of injected electrons may not recombine radiatively with holes and may be wasted via other recombination channels.
In some cases, the above-mentioned problem may be alleviated to a certain extent if the p-type cladding layer is heavily doped. This is because the p-type impurities may be highly diffusive. Even if the p-type impurities are only incorporated during the growth of the p-type cladding layer, they may diffuse into the active region. P-type impurities diffused into the active region may provide additional holes during device operation and enhance the brightness of the device to a certain extent. However, this may not completely solve the above-mentioned problem since, when there are too many quantum wells, the p-type impurities may not be able to diffuse across the entire active region and the wells close to the n-type cladding layer may still lack holes that can recombine radiatively with electrons. Therefore, the brightness of a semiconductor light-emitting device can not be further enhanced. In addition, the p-type impurities diffusing into the active region may exist not only in the barriers but also in the wells and hence form non-radiative recombination centers in the wells. These non-radiative recombination centers may consume part of injected carriers and may be harmful to enhancing the brightness of a device.